The present invention relates to an automatic sheet feeder for use in combination with a facsimile equipment, a copying machine, an optical character reader or the like, for automatically feeding documents, recording sheets or the like to reading unit, a recording unit or a reproducing unit.
A general automatic sheet feeder for use, for example, in combination with an optical character reader is shown in FIGS. 4 and 5. A sheet conveying path is formed between a lower guide plate 1, and an upper guide plate 1' disposed substantially in parallel to the lower guide plate 1. A sheet is conveyed along the sheet conveying path in the direction of arrows. A plurality of sheets stacked in a pile on a feed table, not shown, provided at the entrance of the sheet conveying path are fed one by one intermittently by the cooperative sheet feeding operation of an eccentric first feed roller 2 and a feed spring 15 resiliently pressed against the first feed roller 2. A second feed roller 3 and a highly abrasion-resistant separating rubber plate 16 separate the sheets so that the sheets are fed surely one at a time. A contact, optical image sensing head 6 reads the information recorded on the sheet while the sheet is conveyed along the sheet conveying path by a first conveyor roller 4, first pinch roller 4' pressed against the first conveyor roller 4, a second conveyor roller 5, and a second pinch roller 5' pressed against the second conveyor roller 5.
Although the second feed roller 3 and the separating rubber plate 16 are provided to separate the sheets and to feed the sheets one by one, the second feed roller 3 and the separating rubber plate 16 are unable to surely separate the sheets so that multiple sheet feed is prevented, because the sheet conveying speed is inevitably low to enable the optical image sensing unit 6 to read the information recorded on the sheets. Accordingly, to separate two overlapping sheets, the first conveyor roller 4 is rotated at a circumferential speed higher than that of the second feed roller 3 to convey one of the two overlapping sheets pinched between the first conveyor roller 4 and the first pinch roller 4' at a conveying speed, namely, the circumferential speed of the first conveyor roller 4, higher than the feed speed, namely, the circumferential speed of the second feed roller 3. This speed difference causes the sheet pinched between the first conveyor roller 4 and the first pinch roller 4' to slip relative to the other sheet so that the two sheets are separated from each other. However, since the second feed roller 3 is driven through a feed roller gear 10 at a fixed rotating speed, the sheet conveyed by the first conveyor roller 4 at a high conveying speed rubs on the second feed roller 3, which increases the load on the first conveyor roller 4. To eliminate such a disadvantage, the feed roller gear 10 is mounted through a one-way clutch 9 on the shaft 8 of the second feed roller 3 so that the second feed roller 3 is dragged by the sheet being conveyed by the first conveyor roller 4 at the conveying speed higher than the feed speed.
Nevertheless, in this conventional automatic sheet feeder, the second feed roller 3 feeds the second sheet placed on the first sheet immediately after the first sheet has left the second feed roller 3, so that the first and second sheets are fed successively with a small interval. This small interval causes erroneous detection of the trailing edge or the leading edge of the sheet. Such erroneous detection is a significant problem in certain equipment, such as facsimile equipment, which reads and transmits information recorded on individual sheets.